The invention is in the field of radio frequency mixers for converting a received radio frequency signal to a signal of another frequency and more particularly relates to power efficient mixers for use in small portable apparatus and the like where voltage and current are typically limited by a meagre battery supply.
Mixers, are used for converting a signal of one frequency to another frequency, often referred to as an intermediate frequency signal. The intermediate frequency signal is usually intended for selective filtering in a pass band filter, preparatory to, for example, further processing, detection or amplification. In a so called superheterodyne radio receiver for example, from among received signals a signal of a frequency of interest is selected by mixing the signals with an appropriate frequency signal, from another signal source usually an adjustable local oscillator. A mixer includes a nonlinear element such as a diode or a transistor biased to operate in a nonlinear portion of its operating characteristics. A balanced mixer has the advantage of balancing out the local oscillator frequency and the received signal frequencies leaving only the sum and difference frequencies to be filtered.
One example is disclosed in U.S. Pat. No. 4,636,663 issued Jan. 13, 1987 to A. Jongpier et al. A double balanced RF mixer circuit includes two differential amplifiers each of which comprises a pair of transistors the emitters of which are connected in common and the bases of which are cross-coupled to provide common first and second base terminals to which an oscillator signal is applied. A radio frequency signal to be mixed is applied to a voltage-current converter comprising a pair of transistors connected as a balanced common base circuit, the collectors of such transistors being coupled to the common-emitter terminals of the differential amplifiers.
A more recent example is disclosed in U.S. Pat. No. 5,805,987 issued on Sep. 8, 1998 to F. Kamase and titled Double Balanced Mixer Circuit With Less Power Consumption. The double balanced mixer circuit includes two single balanced mixer circuits each of which has a pair of first transistors. Output side terminals of the first transistors are cross-coupled between the two pairs, and first differential signals are supplied to control terminals of the first transistors. Connected in series to each pair of first transistors is a second transistor of a pair of second transistors. The double balanced mixer circuit also includes a differential amplifier circuit including a pair of third transistors, with fourth transistors connected to the pair of third transistors. The fourth transistors function as constant current sources for the pair of third transistors. Second differential signals are supplied to control terminals of the third transistors and differential output terminals of the third transistors are directly coupled to control terminals of the pair of second transistors. The reduced power consumption of the Kamase mixer is achieved with pluralities of active devices, the transistors, being series coupled. A minimum voltage required to satisfactorily operate the mixer is determined by a sum of the minimum collector emitter voltage drops in the greatest number of transistors chained or coupled in series.
The typical power source for present day wireless radio devices, such as cell phones, personal communicators and the like, is preferably of low voltage. Energizing current is usually supplied with a potential of not more than about 3 volts, however such low voltage is less than convenient for the operation of the typical balanced mixer circuit. The dynamic range of the typical balanced mixer operated with such a low supply potential is reduced such as to be impractical. A greater operating potential for the balanced mixer can ameliorate this problem but with consequential added expense and inefficiency of some additional element such as a DC to DC inverter for example.
It is an object of the present invention to provide a mixer of improved functional linearity while operating with a low voltage power supply as is typically used in a wireless radio device.
In accordance with the invention, a mixer is provided with a single ended input port for receiving radio frequency signals, a balanced input port for receiving local signals and power and ground terminals for energizing the mixer from a low voltage power source. The mixer operates upon input signals, received via the single ended input port and being of desired frequencies, to convert the input signals to output signals with frequencies in a predetermined frequency band. The mixer comprises:
a balanced mixer circuit connected with the power terminal for having energizing current conducted therethrough, connected with the balanced input port for receiving the local signals, and having a pair of input terminals for receiving substantially balanced input signals and a pair of output terminals for providing the output signals;
a differential driver circuit including a first transistor with a collector electrode connected with one of the pair of input terminals of the balanced mixer, a base electrode coupled via a capacitor to the single ended input port, and an emitter electrode coupled via a first resistor to the ground terminal, and a second transistor with a collector electrode connected with the other of the pair of input terminals of the balanced mixer, a base electrode coupled via a capacitor to one of the ground terminal, an emitter electrode coupled via a second resistor to the single ended input port, a fourth resistor connected between the first voltage tap and the base electrode of the second transistor and, a fifth resistor connected between the second voltage tap and the base electrode of the first transistor;
biasing circuit including said first and second voltage taps for providing first and second potentials, the first potential being intermediate a voltage at the power terminal and the second potential and the second potential being at least sufficiently removed from ground for causing a forward current flow between the base and emitter electrodes of the first transistor; and
an impedance element connected between the single ended input port and the ground terminal.
In one example of the invention the impedance element is a resistor having an ohmic value suitable for terminating signals received via the single ended input port.
In another example of the invention the impedance element is an active device connected in series with a resistor and responsive to a potential at the second voltage tap for conducting current.
In yet another example of the invention the impedance element is a parallel resonate circuit having a highest impedance in a band of frequencies which include said desired frequencies.